CS301ES: ANALOG AND DIGITAL ELECTRONICS

DEPARTMENT OF
ELECTRONICS AND COMMUNICATION
ENGINEERING
Instructor
Mr. D V S RAMANJANEYULU
Assistant Professor
Accredited by NBA & NAAC with “A” Grade
CS301ES : ANALOG & DIGITAL ELECTRONICS

P.KIRAN KUMAR,ECE DEPARTMENT 2
UNIT - I : Diodes and Applications
UNIT – II : BJTs
UNIT – III : FETs and Digital Circuits
UNIT – IV : Combinational Logic Circuits
UNIT – V : Sequential Logic Circuits

•Integrated Electronics: Analog and Digital
Circuits and Systems, 2/e, Jaccob Millman,
Christos Halkias and Chethan D. Parikh, Tata
McGraw-Hill Education, India, 2010.
•Digital Design, 5/e, Morris Mano and Michael
D. Cilette, Pearson, 2011
TEXTBOOKS:

UNIT - I : DIODES AND APPLICATIONS
Diodes and Applications:
Junction diode characteristics: Open circuited p-n
junction, p-n junction as a rectifier, V-I characteristics,
effect of temperature, diode resistance, diffusion
capacitance, diode switching times, breakdown diodes,
Tunnel diodes, photo diode, LED.
Diode Applications:
Clipping circuits,
Comparators,
Half wave rectifier,
Full wave rectifier, Rectifier with capacitor filter .

A Diode is the simplest two-terminal electronic
device. It allows current to flow only in one direction and
blocks the current that flows in the opposite direction.
The two terminals of the diode are called as anode
(+) and cathode (-).
DIODE
A K
Symbol of a Diode

The name diode is derived from “Di–Ode”
which means a device that has two electrodes.
Di – means Two (2)
Ode – means Electrodes
Diode: “Di – Ode”

Formation of a Diode
If a P-type and an N-type material are brought close to each
other, both of them join to form a junction.
As shown in the figure below.

P-type material has holes as the majority carriers and
an N-type material has electrons as the majority carriers.
As opposite charges attract, few holes in P-type tend to go
toN-side, whereas few electrons in N-type tend to go to P-
side.
As both of them travel towards the junction, holes and
electrons recombine with each other to neutralize and forms
ions.
Now, in this junction, there exists a region where the
positive and negative ions are formed, called as PN junction
or junction barrier

Diode’s Three Operation Regions
• In order to understand the operation of a diode,
it is necessary to study its three operation
regions: equilibrium, reverse bias, and forward
bias.

The formation of negative ions on P-side and
positive ions on N-side results in the formation of a
narrow charged region on either side of the PN
junction. This region is now free from movable
charge carriers.

The ions present here have been stationary and
maintain a region of space between them
without any charge carriers. As this region acts
as a barrier between P and N type materials, this
is also called as Barrier junction. This has
another name called as Depletion
region meaning it depletes both the regions.

Types of Diode

The PN junction diode is a two terminal device, which is
formed when one side of the PN junction diode is made
with p-type and doped with the N-type material.
Forward & Reverse Biased

Current-Voltage Relationship
Forward Bias: current exponentially
increases.
Reverse Bias: low leakage current equal
to ~Io
Ability of pn junction to pass current in
only one direction is known as
“rectifying” behavior.
PN Junction: I-V Characteristics

Effects of Temperature on V-I Characteristics
In the forward bias , it's shifts to 2.5mV per °C
In the reverse biased condition because the reverse
saturation current of a silicon diode doubles for every
10°C rose in temperature

Diode resistance
Static Resistance or (DC resistance)
Forward Resistance Rf
Reverse Resistance Rr
Dynamic Resistance or (AC resistance)
Forward Resistance rf
Reverse Resistance rr

Diode resistance
• ΔV/ΔI is called ac (dynamic) resistance of the diode because we
consider small change in voltage
• We would not want to calculate ac resistance between V=0.55V and
V=0.65V
• rd= ΔV/ΔI ohms
• The dc resistance of a diode is found by dividing the dc voltage
across it by dc current through it. DC resistance also called the static
resistance.
Rd=V/I ohms
• Diode is nonlinear in both the dc & the ac sense, that is, both its dc
& ac resistance change over a wide range.

AC & DC Resistance
V (V)
I (mA)
0 0.1 0.60.50.40.30.2
0
40
10
20
30
ΔI
ΔV
rD = ΔV / ΔI
RD = V / I
AC ResistanceDC Resistance
rD = VT / I

Diode capacitance
•Transition capacitance (CT)
The change of capacitance at the depletion
region can be defined as the change in
electric charge per change in voltage.
CT = dQ / dV
C = ε A / W
Where,
CT = Transition capacitance
dQ = Change in electric charge
dV = Change in voltage
The transition capacitance can be
mathematically written as,

•Diffusion capacitance or Storage capacitance (CD)
,
•CD is due to the storage of minority
carriers in a forward biased diode
•It will dominate in the device only during
high frequency operation
•CD>CT

switching characteristics of pn junction diode
The sudden change from forward to reverse and from reverse to forward bias, affects
the circuit. The time taken to respond to such sudden changes is the important criterion
to define the effectiveness of an electrical switch.
•The time taken before the diode recovers its steady state is called as Recovery
Time.(trr)
•The time interval taken by the diode to switch from reverse biased state to forward
biased state is called as Forward Recovery Time.
•The time interval taken by the diode to switch from forward biased state to reverse
biased state is called as Reverse Recovery Time.
Storage time − The time period for which the diode remains in the conduction
state even in the reverse biased state, is called as Storage time.
Transition time − The time elapsed in returning back to the state of non-
conduction, i.e. steady state reverse bias, is called Transition time.

Reverse recovery time (trr)= Storage time(Ts)+Transition time (Tt)

Ideal vs practical diode

Block diagram of a Power Supply

Rectifiers
• Rectifier is a device which convert AC
voltage in to pulsating DC
• A rectifier utilizes unidirectional
conducting device Ex : P-N junction
diodes

Types
• Depending up on the period of conduction
 Half wave rectifier
 Full wave rectifier
• Depending up on the connection procedure
Bridge rectifier

Half wave Rectifier
• The process of removing one-half the input signal to
establish a dc level is called half-wave rectification.
• In Half wave rectification, the rectifier conducts current
during positive half cycle of input ac signal only.
• Negative half cycle is suppressed.

Full Wave Rectifier
Circuit has two diodes D1 , D2 and a centre tap
transformer.
During positive half cycle Diode D1 conducts and during
negative half cycle Diode D2 conducts.
It can be seen that current through load RL is in the same
direction for both cycle.

Full Wave Bridge Rectifier
Need for centre tapped PT is eliminated.
Consists of 4 diodes instead of 2.

Full Wave Bridge Rectifier
During period t=0 to t=T/2 D2 and
D3 are conducting while D1 and D4
are in the “off” state.

During period t=T/2 to t=T D1 and D4 are
conducting while D2 and D3 are in the “off”
state.

Filters
 A capacitor is added in parallel with the
load resistor of a half-wave rectifier to
form a simple filter circuit. At first there
is no charge across the capacitor
 During the 1st quarter positive
cycle, diode is forward biased, and C
charges up.
 VC = VO = VS - V.
 As VS falls back towards zero, and
into the negative cycle, the
capacitor discharges through the
resistor R. The diode is reversed
biased ( turned off)
 If the RC time constant is large, the
voltage across the capacitor
discharges exponentially.

Filters
 During the next positive cycle of the input
voltage, there is a point at which the input
voltage is greater than the capacitor
voltage, diode turns back on.
 The diode remains on until the input
reaches its peak value and the
capacitor voltage is completely
recharged.

Quarter cycle;
capacitor
charges up
Capacitor discharges
through R since diode
becomes off
Input voltage is greater
than the capacitor
voltage; recharge before
discharging again
VC = Vme – t / RC
Since the capacitor filters out a large portion of the sinusoidal signal, it is called a
filter capacitor.
NOTE: Vm is the peak value of the capacitor voltage = VP - V
Vp
Vm

Figure: Half-wave rectifier with smoothing capacitor.
Ripple Voltage, and Diode Current
Vr = ripple voltage
Vr = VM – VMe -T’/RC
where T’ = time of the
capacitor to discharge to its
lowest value
Vr = VM ( 1 – e -T’/RC )
Expand the exponential in
series,
Vr = ( VMT’) / RC
T’
Tp

• If the ripple is very small, we can approximate T’ = Tp
which is the period of the input signal
• Hence for half wave rectifier
Vr = ( VMTp) / RC
 For full wave rectifier
Vr = ( VM 0.5Tp) / RC

DIODE CLIPPERS
Clipping circuits basically limit the amplitude of the input signal either below or
above certain voltage level. They are referred to as Voltage limiters, Amplitude
selectors or Slicers. A clipping circuit is one, in which a small section of input
waveform is missing or cut or truncated at the out put section.
Clipping circuits are classified based on the position of Diode.
1.Series Diode Clipper
2.Shunt Diode Clipper

Series Diode clippers

Shunt diode clippers

40
In electronics, a comparator is a device that compares
two voltages or currents and outputs a digital signal indicating which is larger.
It has two analog input terminals V+ and V- and one binary digital output .
Series diode clipper with bias

Slicer (Clipping at two independent levels)

P.KIRAN KUMAR,ECE DEPARTMENT 43P.KIRAN KUMAR,ECE DEPARTMENT 43
Diode equivalent circuits / models:

P.KIRAN KUMAR,ECE DEPARTMENT 44P.KIRAN KUMAR,ECE DEPARTMENT 44
Applications
There are many applications in which diode switching circuits are used, such as −

Breakdown Mechanisms in a diode
 When reverse voltage increases beyond certain value, large diode
current flows, this is called breakdown of diode, and corresponding
voltage is called reverse breakdown voltage of diode.
 There are two distinct mechanisms due to which the break down
may occur in the diode, these are:
•Avalanche breakdown
•Zener break down

Avalanche Breakdown:
The avalanche breakdown occurs in lightly doped diodes.
The multiplication factor due to the avalanche effect is given by
1
1
M n
V
VBD


 
 
 
 
Where M is carrier multiplication factor
n-type silicon n=4 and For p-type n=2
V is applied reverse voltage
VBD is reverse breakdown voltage

Zener Breakdown
•The zener breakdown occurs in heavily doped diodes.
•For heavily doped diodes, the depletion region width is small.
•Under reverse bias conditions, the electric field across the depletion layer is very
intense. Breaking of covalent bonds due to intense electric field across the narrow
depletion region and generating large number of electrons is called Zener effect.
•These generated electrons constitute a very large current and the mechanism is
called Zener breakdown.
•The diodes having reverse breakdown voltage less than 5v shows the Zener
mechanism of breakdown.

Zener Diode
•Zener diode is a heavily doped diode, and is designed with adequate power
dissipation capabilities to operate in the reverse breakdown region.
•The operation of the zener diode is same as that of ordinary PN diode under
forward biased condition.
•In reverse biased condition, the diode carries reverse saturation current, till the
reverse voltage applied is less than the reverse breakdown voltage.
•When the reverse voltage exceeds the reverse breakdown voltage, the current
through it changes drastically but the voltage across it remains almost constant such
a break down region is a normal operating region for a zener diode.
The symbol of zener diode is

Zener Diode
The dynamic resistance of a zener diode is
defined as the reciprocal of the slope of the
reverse characteristics in zener region.
Vzrz
Iz



= 1 / slope of reverse
characteristics in zener region
•The dynamic resistance is very small, it is of he order of few tens of ohms.

Equivalent circuit of Zener diode

Applications of zener diode
The various applications of zener diode are,
•As a voltage regulating element in voltage regulators.
•In various protecting circuits.
•In zener limiters i.e., clipping circuits which are used to clip off the unwanted
portion of the voltage waveform.

Tunnel diode
•If the concentration of impurity atoms is greatly increased, say 1 part in 103
the device characteristics are completely changed.
•The new diode was announced in 1958 by Leo Esaki. This diode is called
‘Tunnel diode’ or ‘Esaki diode’.
•The barrier potential VB is related with the width of the depletion region with
the following equation.
•From the above equation the width of the barrier varies inversely as the
square root of impurity concentration.
•As the depletion width decreases there is a large probability that an electron
will penetrate through the barrier. This quantum mechanical behavior is
referred to as tunneling and hence these high impurity density pn-junction
devices are called Tunnel diodes.
q NAV . ²B 2


B2V
² .
q NA


 

Tunnel diode
Energy band structure of heavily doped pn-junction diode under open
circuited conditions

The volt-ampere characteristics
Under applied reverse bias

Under applied Forward bias

Under applied reverse bias

Tunnel diode
The tunnel diode symbol and small-signal model are
Applications of Tunnel diode:
•It is used as a very high speed switch, since tunneling takes place at the speed of
light.
•It is used as a high frequency oscillator.

Photodiode
•The photodiode is a device that operates in reverse diode.
•The photodiode has a small transparent window that allows light to strike
one surface of the pn-junction, keeping the remaining sides unilluminated.
The symbol of photodiode is

The volt-ampere characteristics of photodiode

Photodiode
Advantages of Photo diodes:
•It can be used as variable-resistance device.
•Highly sensitive to the light.
•The speed of operation is very high.
Disadvantages of Photo diodes:
The dark current is temperature dependent.
Applications of photodiode:
Photodiodes are commonly used in alarm systems and counting systems.
Used in demodulators.
Used in encoders.
Used in light detectors.
Used in optical communication systems.

Light Emitting Diode (LED)
The LED is an optical diode which emits light when forward biased, by a
phenomenon called electroluminescence.
The LEDs use the materials like Gallium Arsenide (GaAs), Gallium
Arsenide Phospide (GaAsP) or Gallium Phospide (GaP). These are the
mixtures of elements Ga,As,P.
The symbol of LED is

LED Working Principle
•When an LED is forward biased, the electrons and holes move towards the
junction and recombination takes place.
• As a result of recombination, the electrons lying in the conduction bands of
n-region fall into the holes lying in the valance band of p-region.
•The difference of energy between the conduction band and the valance
band is radiated in the form of light energy.
•The energy released in the form of light depends on the energy
corresponding to the forbidden gap. This determines the wavelength of the
emitted light.
•The wavelength determines the color of the light and also determines
whether the light is visible or invisible (infrared).

LED Working Principle
•The color of the emitted light depends on the type of material used.
Gallium Arsenide (GaAs) --- Infrared radiation (invisible)
Gallium Phospide (GaP) --- Red or Green
Gallium Arsenide Phospide (GaAsP) --- Red or Yellow.
•The brightness of the emitted light is directly proportional to the forward
bias current.

Output characteristics of LED
Typical output characteristics for LED
Process of electro luminescence

Advantages of LEDs:
• LEDs are small in size.
•LEDs are fast operating devices.
•LEDs are light in weight.
•LEDs are available in various colors.
•The LEDs have long life.
•The LEDs are cheap and readily available.
•LEDs are easy to interface with various other electronic circuits.
Disadvantages of LEDs:
• Needs large power for the operation.
•The characteristics are affected by the temperature.

Applications of LEDs:
• The LEDs are used in all kinds of visual displays i.e., seven
segment displays and alpha numeric displays. Such displays
are commonly used in multimeter, calculator, watches etc.
•LEDs are also used in optical devices such as optocouplers.
They are also used in burglar alarm systems.

CS301ES: ANALOG AND DIGITAL ELECTRONICS

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